The present invention relates to a modulator for use in plasma immersion ion implantation (PIII), such a modulator being an essential component for a PIII installation.
For PIII, a workpiece which is to be implanted is immersed in plasma within an implantation chamber. By applying a high negative voltage to the workpiece, positive ions are accelerated out of the plasma onto the workpiece and implanted in the latter. (J. R. Conrad, J. L. Radtke, R. A. Dodd, F. J. Worzala and Ngoc C. Tran, Plasma source ion implantation technique for surface modification of materials, Journal of Applied Physics, 62 1987, 4591-4596). The current density of the implanted ions depends on the plasma parameters and on the implantation voltage selected and is of the order of 1-10 mA/cm.sup.2 during the implantation. At an acceleration voltage of 50 kV, the energy input ranges therefore up to 500 W/cm.sup.2. Since this energy input would lead to strong heating of the workpiece after a few seconds, the high negative voltage is pulsed and not applied continuously to the workpiece.
A further reason for pulsing the high voltage is the fact that the current required for larger workpieces can be maintained over a prolonged period of time only with appreciable technical effort. The pulse duration is between 2 and 100 .mu.s at a repetition frequency of a few Hz up to kHz.
Accordingly, an important component of a PIII installation is the so-called modulator which modulates or regulates the appropriate high voltage pulses from a source of rectified high voltage to the workpiece which is to be implanted.
The construction and mode of functioning of the modulators used until now for a PIII installation are always similar. The positive electrode of a high voltage generator is connected via a resistor to a capacitor, the second terminal of which is connected via a high voltage lead-through to the processing table, which is situated within the implantation chamber and carries the workpiece that is to be implanted. The other electrode of the high voltage generator is connected as ground potential 0 to the implantation chamber. Moreover, a switch is disposed at a connection point between the resistor and the capacitor and enables the ground potential to be connected to this pole of the capacitor. The switch controls the charge/discharge of the capacitor, and through the switch, the high voltage pulsing is applied to the workpiece. Since this switch must be able to switch high voltages of several 10 keV at, in some cases, appreciably high currents, switches, which are commercially available at the present time and meet the requirements of PIII, also represent an appreciable cost factor. For smaller installations, especially those designed for research purposes, relatively inexpensive switches, based on semi-conductors, can still be used. With these switches, implantation voltages up to 40 kV and currents up to the ampere range can be switched. If larger currents are to be switched, expensive transmitting tubes, which are attainable, for example, for voltages up to 40 kV and currents up to 200 ampere, must be used. Still higher currents up to 1 kA can be switched with the so-called "Crossatrons" (D. M. Goebel, High power modulator for PII, J. Vac. Sci. Technol. B. Vol. 12, 2, 1994, 838-842).
A further increase in the currents to be connected is possible only with extremely expensive custom-made products, such as those known from fusion research (B. J. Wood, J. T. Scheuer, M. A. Nastasi, R. H. Olsher, W. A. Reass, I. Henins and D. J. Rej, Design of a large-scale plasma source ion implantation experiment, Materials Research Society Symposium Proceedings, V 279, 1993, Materials Research Society, Pittsburgh, Pa., USA, 345-350).